The stabilisation and aggregation of colloidal dispersions or emulsions of particles in water or in aqueous solutions, has been explained in terms of DLVO theory (an acronym for the workers Derjaguin, Landau, Verwey and Overbeek who developed the theory) which combines the effects of van der Waals attraction with electrical double layer repulsion between dispersed, charged colloidal particles.
Commonly charged colloidal particles (i.e. colloidal particles having the same sign of charge) are stabilised in colloidal dispersions by mutual electrostatic repulsion forces exceeding the attractive van der Waals attraction. Particles in a colloidal state may typically have a particle diameter from about 1 to 10,000 nm.
The charged particles may attract counterions, of opposite charge to their charged surfaces, from their aqueous surroundings, resulting in the formation of an electrical double layer (EDL) at the particle surface. This EDL screens the electrical repulsion between particles, and so by formation of a suitable EDL, the electrostatic repulsion between the commonly charged colloidal particles may be sufficiently screened in order to allow van der Waals forces to drive coalescence of the particles into larger, bulk agglomerates or flocs.
Typically, for water purification, or for extraction of desired materials from an aqueous dispersion or slurry, in order to remove colloidal particles from water by flocculation, modification of the EDL may be achieved by addition of electrolyte to the colloidal dispersion to be flocculated. However, for water purification, this has the disadvantage that high levels of dissolved electrolyte may remain in the water remaining after flocculated material has been removed.
Electrocoagulation is based upon the use of electrochemical dissolution of an electrode by electrolytic oxidation with OH− to form counterions of high charge, at the anodes, which can aid flocculation (typically cations such as Fe3+ or Al3+ for flocculation of fatty particles) without the need for addition of corresponding salt-derived anions into the liquid to be treated (typically OH− will be the counterions formed in the electrocoagulation process). In parallel with the formation of the cations formed at the anode, gas bubbles (hydrogen) are also formed at the cathode.
For a typical electrocoagulation system, opposed electrodes may be used to provide a voltage difference across one or more sacrificial electrodes positioned between the opposed electrodes, with the sacrificial electrodes not electrically connected to each other or to the opposed electrodes other than through the liquid. This results in an electrical field being set up across the sacrificial electrodes, causing them to have cathodic and anodic surfaces and causing a current to flow between them and the opposed electrodes, typically with the material of the sacrificial electrodes oxidising and dissolving at the anodic surfaces and hydrogen bubbles being generated at the cathodic surfaces. For instance with sacrificial electrodes of aluminium, aluminium hydroxide is formed at the cathode and can lead to flocculation or co-precipitation of colloidal particles within the liquid to be treated.
For removal of dispersed particulate matter from water, the presence of gas bubbles from the cathode, subsequently entrained within the resulting floc of particulate matter, may assist in removal of the particulate matter by flotation and bulk separation, as the particulate matter, particularly if fatty matter, is typically of lower density than water, and the additional presence of entrained gas bubbles may further reduce the density of the floc formed, assisting in speeding separation by flotation of the floc to form a separate layer for subsequent removal to leave purified water.
A problem with electrocoagulation systems is the need to replace the electrodes at intervals as the electrodes dissolve or become coated with impervious oxide layers during use. Whilst electrodes are being replaced, the liquid to be treated, such as waste water to be purified, may continue to accumulate and so there is a need to provide electrocoagulation apparatus and methods which allow for easy and rapid replacement of electrodes in use.